The region of transcriptional activity in the rat LH receptor gene has been localized within the 5' flanking 173 bp domain adjacent to the initiation codon in expressing and non-expressing cells. Repression of this promoter activity in both cell types was induced by upstream elements in at least two domains. Am additional upstream inhibitory element present only in non-expressing cells is likely to participate in constitutive repression of LH receptors. Analysis of the 173 bp domain revealed two independent promoter regions. The first domain between -1 and -137 bp carries by typical TATA or CAAT boxes, but does contain SP1 elements and inhibitor sites. A second promoter domain between nucleotides -120 and -173 is predominant in non-expressing cells. Tissue-specific DNA binding proteins, observed only in expressing cells, are present for each of the promoter domains and may be important in tissue-specific regulatory events. A protein common to both expressing and non-expressing cell types (C-box protein) may play a central role in basal transcription. Complex regulatory patterns in LH receptor gene transcription can reflect the alternate activation of these two promoter domains, and may be involved in hormone-induced fluctuations of gene expression during gonadal maturation. Lactogen receptor heterogeneity in the gonads has been defined at the molecular level by the characterization of ovarian cDNA clones coding for distinct receptor isoforms (clone R1, coding for 80 kDa transmembrane form with short cytoplasmic domain; L1, for a 40 kDa transmembrane with long cytoplasmic domain; and R2, a soluble receptor form). Differential hybridization of ovarian mRNA demonstrated that the long form is derived from the 9.7 and the 4.6 mRNA species. Two distinct mRNA species (1.8 and 2.1 kb) correspond to the short transmembrane form with differences their non- coding regions. A 2.6 kb mRNA (not yet cloned or the protein isolated) is a variant of the long transmembrane form with a truncated or altered extracellular domain, and could be a constitutively active gene product. The 9.7 kb species is the only species observed in early development, while the other forms are induced by FSH or LH. All five mRNA species are expressed and regulated in a developmental stage-specific manner during gonadotropin-induced ovarian maturation, luteinization, and desensitization. Coordinate up- and down-regulation of receptor binding activities and mRNA levels by gonadotropin suggest that a common mechanism is involved in the regulation of the two receptor mRNA's.